The invention is particularly concerned with methods of operating a controllable multi-variable process and systems for use in the operation of such a process. Methods and systems of this kind are described in GB-A-2 363 647 and GB-A-2 378 527 in which a multi-dimensional display representation of the variables is derived according to individual, parallel or other spaced coordinate axes. The variables of the process are of two kinds, namely, process-variables, of which the current values are generally-available during process operation, and quality-variables, of which the values are generally determined later by laboratory or similar measurement or analysis. A boundary or envelope for prospective operation of the process is defined within the system of axes in accordance with sets of values for the process- and quality-variables accumulated as a historical record of a multiplicity of previous, satisfactory operations of the process. Each previous operation is defined as a respective datapoint consisting of a set of values for the individual process- and quality-variables applicable to that operation.
The method and system described in GB-A-2 378 527 involves the calculation and use of an envelope termed the BOZ, representing the best operating zone of the process. The BOZ is calculated from a selected, limited group of the historical record of datapoints so the envelope of the BOZ lies wholly within the outer, boundary envelope derived from the full historical record. The two envelopes are used for different purposes in the known methods and systems, the outer envelope for the purpose of revealing the region within which operation of the process may take place satisfactorily within the bounds of past experience. The envelope of the BOZ, on the other hand, establishes the ranges of the individual process-variables that are applicable to realisation of ‘good’ results and provides a prediction of the ranges of the quality-variables that can be expected to be achieved.